supercub,
If I'm reading your post correctly, you only NEED to add 30' to what is already there to make the 100'. So the posts above about the weight of a 100' cable are not relevant. Your current cable is 30', so you already know what a 30' cable weighs. :-)
As far as being doable, in general, I would say yes. It DOES depend on the voltage at the house. If it's in the normal acceptable range, there shouldn't be any problem. However, if it's low (either due to power utility supply, poor or old wiring in the house or localized "brown-outs"), then you'll have to be watchful of the voltage.
You can purchase plug-in voltmeters you can plug into your RV to monitor the voltage when both ACs are needed. Get two and plug each one into an outlet on each leg of the supply.
Someone brought up the point that both ACs might be on the same leg. If that's the case, I would STRONGLY recommend that one of them be moved to the opposite leg. That way, when BOTH ACs are running, the current through the neutral wire will basically be ZERO for the ACs. So you won't have any cable heating with a heavy current down the neutral. If both ACs are on the same leg, the neutral current will be the COMBINED current of the two ACs. If they're on opposite legs, the neutral current will be the DIFFERENCE between the two ACs. Assuming the ACs are similar, then that would be about zero.
Perhaps that was too much info? Sorry if it was.
By the way, I've routinely used 70' of cable at my brother's house without any issue. My RV has a 30' cord and I added a 15' and 25' extension cord. Again, if the voltage is proper at the homestead, I don't think you'll have any problem.
Good Luck, and let us know how you make out!
~Rick
